Problems Linking Airport Express Stations to Airport Exreme Base Station



I recently moved into a new apartment where I am trying to set up a wireless network using an airport extreme base station and three airport express stations. Using just the airport extreme base station, I get a faint internet signal throughout the apartment (the apartment is long and narrow, so it doesn't surprise me that I'm not able to get a strong signal). However, when I add the airport express stations to the network using the Airport Setup Assistant utility and select 'extend wireless network' option, the internet connection actually DEGRADES: suddenly, I'm barely able, or unable, to get an internet connection at all. I have used the airport express station in question on its own as a base station at my old apartment, so I'm sure that the device is not defective.

Can you tell me why adding an express station as a remote station to the network would actually worsen the internet connection, and how I might fix this? Presently, after investing hundreds of dollars in the extreme station and base stations I have very poor internet connectivity and virtually no iTunes connectivity to speak of (when I try to stream iTunes to any of my base stations, the connection cuts out constantly).

Thank you!